Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the “skin” of the conductor, between the outer surface and a level called the skin depth (δ) as shown in prior art FIG. 1. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor. For alternating current, nearly two thirds of the electrical current flows between the conductor surface and the skin depth, δ. The skin effect is due to opposing eddy currents (Iw) induced by the changing magnetic field (H) resulting from the alternating current (I) as shown in prior art FIG. 2. For example, at 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth becomes much smaller and increases AC resistance.
A proximity effect occurs in an AC carrying conductor, where currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, and the distribution of current within the first conductor is constrained to smaller regions. The resulting current crowding is termed the proximity effect. The proximity effect increases the effective resistance of a circuit, which increases with frequency. As was explained above for the skin effect for AC flow, the changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor, by electromagnetic induction. When an alternating current (AC) flows through an isolated conductor, the alternating current creates an associated alternating magnetic field around it. The alternating magnetic field induces eddy currents in adjacent conductors, altering the overall distribution of current flowing through them. The result is that the current is concentrated in the areas of the conductor furthest away from nearby conductors carrying current in the same direction. Similarly, in two adjacent conductors carrying alternating currents flowing in opposite directions, such as are found in power cables and pairs of bus bars, the current in each conductor is concentrated into a strip on the side facing the other conductor
In order to address transmission loses and inductance associated with transmission associated with the skin effect, the prior art has often resorted to numerous thin conductors that form a bundle as shown in FIG. 3. This has not been wholly successful in that electromagnetic effects are non-uniform across the bundle cross-section thereby creating other types of transmission loses.
FIG. 4 illustrates a prior art, existing cable design 10 formed of several insulated conductor wires (14, 16) in an interwoven pattern 12 and grouped into like bundles of conductors (14b, 16b) at the cable input and output terminations 18. While this design offers an improved operating performance, non-uniform heating still results during operation due to variations in conductor wire lengths in the weave pattern.
While there have been many advances in electrical transmission cable design, there still exists a need for electrical transmission cables with low inductance properties capable of carrying high current loads with a more uniform heating or loss profile.